import java.util.Scanner;

public class shuifei 

{
  
	public static void main (String args[])
  
	{
    
		Scanner reader = new Scanner (System.in);
		
double x,y;
    
		x=reader.nextDouble();
    
		if (x<=15)
    
		{
      
			y=4*x/3;
    
		}
    
		else
    
		{
      
			y=2.5*x-17.5;
    
		}
    
	System.out.printf("%.2f",y);
  
	}

}